With DC-DC converters placed into wide use in recent years, it has been required to provide such converters which are improved in characteristics, especially in efficiency, reduced in noise, lower in cost and smaller in size and weight. DC-DC converters of the type mentioned are lower in noise and smaller in size and weight than those of other types, and are introduced into use for various applications (T. IEE Japan, Vol. 111-D, No. 12, 1991, pp. 1087-1096).
With reference to FIG. 3, a description will be given first of the circuit of a conventional DC-DC converter of the current resonance type having a single switch device.
Between the primary inductance 4 of a high-frequency transformer HT and the secondary inductance 9 thereof, this circuit can be divided generally into a primary circuit including a DC power supply 1 and a secondary circuit including a load 15. The secondary circuit has a bridge rectifier circuit which is substituted for a full-wave rectifier circuit comprising a center-tapped transformer and used on the secondary side of the converter of the above literature.
The primary circuit comprises an input smoothing capacitor 2 connected in parallel to the DC power supply 1, and a zero-current switching resonance switch circuit (hereinafter referred to as the "ZCS resonance switch circuit 3") connected at its input side to the capacitor 2. The output side of the ZCS resonance switch circuit 3 is connected to one end of the primary inductance 4 of the high-frequency transformer HT.
The switch circuit 3 has a resonance inductance 5 serving as an input device. The inductance 5 for resonance has one end connected to the positive terminal of the DC power supply 1 and the other end connected to one end of a capacitor 6 for resonance and to the above-mentioned one end of the transformer primary inductance 4. The other ends of the resonance capacitor 6 and the transformer primary inductance 4 are connected to one end of a semiconductor switch device 7 and the cathode of a diode 8. The other end of the switch device 7 and the anode of the diode 8 are connected to the negative terminal of the power supply 1 and to the input smoothing capacitor 2.
On the other hand, the secondary circuit comprises a rectifier bridge of diodes 10 having AC input terminals connected to opposite ends of the secondary inductance 9 of the high-frequency transformer HT, and a choke coil 11 having one end connected to the positive output side of the diode bridge 10 and the other end connected to one end of each of a smoothing capacitor 12 and the load 15. The other ends of the smoothing capacitor 12 and the load 15 are connected to the negative side of the diode bridge 10.
The operation of the conventional converter circuit will be described. While the semiconductor switch device 7 is not conducting, the direct current through the primary circuit is blocked at the positions of the switch device 7 and the diode 8, and the resonance capacitor 6 discharged by the primary inductance 4 of the high-frequency transformer HT is held at zero charge level thereacross. The switch device 7 is triggered into conduction by an external control circuit (not shown) and brought out of conduction by the control circuit upon resonance current i.sub.Lr becoming zero when the direction of flow of the current changes from clockwise to counterclockwise. The external control circuit usually monitors the output voltage and produces a trigger of a period in conformity with the output voltage.
When the semiconductor switch device 7 is triggered into conduction by the control circuit, series resonance of the resonance inductance 5 and the resonance capacitor 6 starts to charge the capacitor 6, namely, to apply a voltage across the capacitor 6. At the same time, the resonance current i.sub.Lr partly starts to flow through the primary winding of the transformer HT as a transformer current i.sub.T1. The series resonance circuit exhibits a very low impedance at resonance as is well known, and a great resonance current flows from the DC power supply 1 into the series resonance circuit at resonance since this circuit serves directly as the load of the DC power supply 1.
The resonance current i.sub.Lr due to the series resonance effected by the inductance 5 and the capacitor 6 reduces to zero the moment the direction of flow of the current i.sub.Lr changes from clockwise to counterclockwise, whereupon the switch device 7 becomes nonconducting. The resonance current i.sub.Lr flows counterclockwise through a closed circuit comprising the capacitor 6, inductance 5, DC power supply 1 and diode 8. In this way, one cycle of resonance is completed, and is followed by the next trigger. A portion of the counterclockwise resonance current i.sub.Lr flows also through the transformer primary inductance 4 which is disposed in parallel to the resonance capacitor 5.
During the cycle of resonance, on the other hand, voltage occurs across the transformer primary inductance 4, and the transformer current i.sub.T1 flows therethrough as stated above. This develops an AC voltage across the transformer secondary inductance 9 in proportion to the turn ratio. The AC voltage is rectified by the diode bridge 10 and supplied to a smoothing circuit comprising the choke coil 11 and the smoothing capacitor 12. The smoothing circuit is a low-pass filter which is set to a sufficiently low cut-off frequency relative to the resonance frequency, and removes an AC component to give a substantially steady direct current, which is supplied to the load 15.
With the conventional converter described wherein the resonance current i.sub.Lr has a very great peak value when the semiconductor switch device 7 is conducting, the great resonance current is wholly supplied by the DC power supply 1 and the input smoothing capacitor 2.
Generally, the higher the resonance frequency, the more advantageous is the circuit to compact, so that it is common practice recently to use a resonance frequency which is as high as several tens of kHz to several hundreds of kHz. Usually, the DC-DC converter is separate from the DC power supply 1 and is connected thereto by wiring. Accordingly, the greater the high-frequency resonance current i.sub.Lr flowing through the DC power supply 1, the greater is the likelihood of electromagnetic disturbances occurring. This gives rise to a need, for example, to fortify the input smoothing capacitor 2 disposed in the vicinity of the power input terminal of the DC-DC converter so as to confine the high-frequency component within the circuit. Such a countermeasure presents difficulties in compacting the converter.
Further if a great high-frequency current flows through the DC power supply 1, the internal capacitor of the power supply 1 generates heat, which impairs the life of the capacitor, consequently shortening the life of the power supply 1.
Moreover, the resonance current i.sub.Lr, if great, entails increases in the loss due to the conduction resistance of the semiconductor switch device 7 and in the ohmic loss of the resonance inductance 5 to result in a lower circuit efficiency of about 80%.